The result in many homes is living-space floors with 2-by-10s and attic floors with 2-by-6s. The rational is that attic joists only handle light storage.

Attic fire believed to be electrical in nature Firefighters extinguished flames on the second floor before gaining access to the attic where they found attic joists, roof boards, and insulation ...

Even if your attic is not a good candidate for conversion into full-time living area, there are inexpensive ways to make better use of that extra space.